please feel free to upload your pictures of cats

my personal opinion would be to let it starve for a day or 3 ( making sure to let it have plenty of water), then try its food again. If you are being too much of a "soft parent" with your animal letting it eat meats and fish and you are not an expert nutritionist for animals, then you are doing your cat more harm than good. In other words stop babying your cat. Treat it like the animal it is and stop trying to make it human because it is not human. the prior statements do assume "it's food" as you defined it is a packaged "cat food" product that it has eaten in the past.
if the above does not work, find another vet and/or a nutritionist for the cat. You seem to need an expert in this field if you can't find the issue yourself.
You could also try chicken hearts, available from the supermarket and not expensive. Just try tempting puss with different things until you find what satisfies.